As the deep sub-micron technology improves, the device geometry has been greatly reduced, which leads to the reduction of the operating voltage, as well as the signal amplitude of the circuit. Under such circumstances, noise is becoming more prominent than before. To maintain the signal-to-noise ratio (SNR) under an acceptable level to keep the circuit functions, various solutions have been proposed to reduce the noise appearing in the circuit. This is especially true for the noise appearing in the reference voltage sources.
The conventional techniques to suppress the reference noise are to use a large capacitor and a large resistor on the signal path to form a low-pass pole for filtering the noise. However, in the IC design, a large capacitor and a large resister will occupy a large chip area, which is not economically viable. Therefore, these approaches are rarely used in practice, except in a limited application where integration and monolithic design takes less priority than the needs of the reference noise suppression. Even in those applications, there exists additional problem, such as the large circuit switching on/off delay.
Hakkinen, Rahkonene, and Kostamovaara proposed in the article “An integrated programmable Low-Noise Charge Pump” (Proceedings of ICECS, 1999) a design of an integrated programmable charge pump based on an op-amp current mirror, as shown in FIG. 1. The design is applicable to the charge pump of the lock-phase loop, which has strict requirements in terms of noise level. The prior art uses an op-amp 101 to construct a feedback circuit, and, based on the negative feedback mechanism, the noise of the current source ID is suppressed by the loop gain provided by the op-amp 101. However, the prior art does not suppress the noise in the reference source Vref. The reference noise is converted by a resistor 103 and injected into the circuit.
U.S. Patent Publication 2003/0169872 disclosed a voltage reference filter for subscriber line interface circuit, as shown in FIG. 2. As shown therein, the direct current (DC) component of the reference signal is first filtered by a high-pass RC filter (capacitor 201 and resistor 202). The alternating current (AC) component passes the amplifiers 203, 204, and then a subtracting circuit 205 is used to subtract the AC component from the reference signal to obtain the DC component of the reference signal. However, for the high-pass filter to be effective, the RC value must be sufficiently large, which may even require the use of external elements. In addition, this prior art employs multiple amplifiers and hence consumes a considerable amount of static current. Therefore, although this prior art is applicable to the subscriber line, it is not generally applicable to other systems that demand low manufacturing cost, small device geometry and high integration.
FIG. 3 shows a conventional current mirror. A current mirror is a circuit to copy a current flowing through one active device by controlling the current in another active device of a circuit, keeping the output current constant regardless of the loading. The current being copied can be a varying signal current. Current mirrors also allow current signals to have a fanout greater than one and each output can be scaled using appropriate W/L ratios. Another important function of the current mirror is to reverse the current direction. As shown in FIG. 3, a reference current IREF is mirrored to transistor M2 through transistor M1, then mirrored to transistor M4 through transistor M3, and finally flows to the circuits requiring the current.
Reference current IREF is a fixed current generated by another circuit block, which, in general, is a bandgap reference voltage generator. As shown in FIG. 3, the noise in reference current IREF will flow along with the DC component to be mirrored from M1 to M4, and enters the operating circuit through M4. Along the current flow, more noise current from M1, M2, and M3 will be added. All the noise currents, if not suppressed, will greatly affect the noise characteristic of the operating circuit.
To suppress these noises, a conventional method is to place a large capacitor at the gate of M4, as the capacitor C1 shown in FIG. 3. Capacitor C1 and transistor M3 form a low-pass pole to perform a first-order filtering for the noise. The location of the pole is gm3/C1, where gm3 is the conductance of transistor M3. For example, if gm3 is 0.9 mA/V, a capacitor C1 of 10 pF integrated into an IC can create a 14 MHz pole. However, to filter noise of even lower frequency, the required capacitance will be even larger, which makes the integration even harder.
Another technique is to place a resistor with large resistance between M3 and C1. Similarly, the resistor and C1 form a low-pass pole to perform a first-order filtering for the noise. Although the integrated resistor occupies a smaller area, it still requires a rather large resistance and large capacitance to generate a pole of sufficiently low frequency. Therefore, the overall integrated area is still considerably large.
FIG. 4 shows a simplified block diagram of the operation of a filter used in conjunction with a voltage source. Vin is a source consisting of a DC component and high-frequency noise component. Assume that a very low frequency pole is introduced such that the low-pass filter filters out the high-frequency noise component while allowing the DC component to pass; hence the Vout consists of only the DC component. However, the changes in Vout, for example, on and off, cannot react as immediately as the changes in Vin because of the large time constant introduced by the filter.